This invention is in the field of digital communication receivers and relates to hard and soft bit demapping of Quadrature Amplitude Modulation (QAM) signals employing non-square, Gray coded, constellations.
Recovery of modulating bits in a QAM receiver can be accomplished by symbol level demodulation followed by symbol to modulating bit sequence conversion or by direct bit level demapping. Bit level demapping can be such that the demapper output (information associated with the modulating bits) is ‘hard’, i.e., either a physical relative value of normally +1 unit, for a bit of logical value 1, or −1 unit for a bit of logical value 0. Alternatively, the demapper output can be ‘soft’, a soft-bit indicating how likely the modulating bit associated with a given demapper output is to be of logical value 1 or logical value 0. An example of soft-bits are log-likelihood ratios (LLRs), which have the same sign as that which would have been provided by a hard decision demapper, i.e., + or −, but whose magnitude indicates the level of confidence in that hard demapping decision.
If modulating bits are uncoded, the demapper output is always hard. If modulating bits are coded, via a convolution encoder, for example, the demapper output may be either hard or soft, as the decoder following the demapper can be made to operate either with hard input bits or with soft input bits. The decoder output, on the other hand, is always hard. In general, decoding via soft-bits is much more reliable than that via hard-bits.
In D. Morais U.S. Pat. No. 8,422,579 B1, dated Apr. 16, 2013 and entitled “Quadrature amplitude modulation via modified-square signal point constellation,” new non-square, Gray coded, QAM constellations were introduced. These constellations exhibit a lower peak to average power ratio than their traditional square counterparts. For the traditional square, Gary coded, QAM constellations, low complexity methods exist for hard and soft level bit demapping, enhancing their utility. For the non-square, Gary coded, QAM constellations introduced in U.S. Pat. No. 8,422,579 B1, the availability of low complexity methods for hard and soft level bit demapping would similarly enhance their utility.